1. Field of the Invention
The present invention relates to an optical disk complied with a Blu-ray Disc on which data are recorded using a combination of pits and lands, and a method of producing the optical disk.
2. Description of the Related Art
Read-only ROM (Read on Memory) disks may be mass produced from one stamper by injection molding in a fairly short time, and various data are recorded on the disk including management information or content information such as music and images using a combination of lands and pits simultaneously with molding of the disks.
Accordingly, a large number of replica optical disks called pirated optical disks may be illegally produced with ease from legitimate ROM disks, and hence a method for preventing the illegal production may be required with various manners.
Each of the legitimate optical disks on which main data such as management information or content information are recorded is preferably configured so that secondary data such as ID data based on a code, a mark, or the like may be recorded on the disk in order to certify that the disk is a legitimate optical disk.
In contrast, Japanese Unexamined Patent Application Publication No. 2006-134385 discloses a Blu-ray Disc that is a read-only ROM optical disk on which data such as management information or content information are recorded using a combination of pits and lands, where a mark pre-formed by mastering the disk is irradiated with high output laser light to deform part of the mark, specifically, to deform an irregular pattern of the pits and the lands, so that the ID data or the like are recorded.
In a ROM Blu-ray Disc, a reflective film is formed on an optical disk substrate having a pit array involving main data, and a protective layer is formed on the reflective film. The disk is irradiated with reproduction laser light from a side of the protective layer, so that information is retrieved.
Accordingly, in the Blu-ray Disc, a surface roughness of the reflective film is a noise of a reproduction signal.
The Blu-ray Disc including an Ag alloy to have sufficient optical properties at a blue laser wavelength. This Blu-ray Disc has sufficient optical properties because a reflective film of the disk has a small surface roughness Ra (center line average roughness) and a high reflectance.
However, when the secondary data are recorded in a typical reflective film having Ag by irregular deformation of lands and pits, it is difficult to increase recording sensitivity and to control a recording laser power to 20 mW or less, because the film has high thermal conductivity.
In contrast, Japanese Unexamined Patent Application Publication No. 2003-317318 discloses a ROM optical disk including an Al alloy reflective film in which recording is to be performed.
The Al alloy reflective film is cost effective since that the film is less expensive than an Ag alloy reflective film. However, there has yet been developed a ROM optical disk including the Al alloy reflective film in which surface roughness is controlled, and which has improved recording sensitivity and satisfactory optical properties such as reflection and absorption properties suitable for a Blu-ray Disc in which information is recorded by irregular deformation of lands and pits.